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Late in flower, so it'll probably be fine regardless. Maybe, a cal-mag supplement? Rust spots can be other things too. FF fertilizer can get you through, but a better complete system will do you wonders, even in soil. You won't experience this sort of stuff later on. The soil you have will provide all the nutrients for a fairly long period of time, but will require more at some point (maybe, a 12-12 from see would last? or 1 week of vege, etc. Does depend on length of grow, obvsiouly). Fertilize more later on. Just hydrate early on until you see a slight deficiency of any type, then regular feeding is good. Likely need to slowly ramp up even then. 1/2 times a week with ff is all you ever need or you burn your plants. Don't underwater or overwater. Water it and wait for top 1/2" to 1" to dry. Learn weight of pot when saturated and at this point for one more point of verification. Slight drying of substrate is good, but wilt on plant is not. Ca deficiencies take 60 days to present themselves. By the time you see it, it was occurring long ago. You can definitely stagger harvest, if you want. If it's the ripeness you want, cut it off. I'd do this over a short window of time to reduce chance of nanners from stress, though. 2-3 days isn't enough time to fertilize anything even if it starts to grow a nanner. I rather just keep things timed to gether, keeps it simpler for drying/curing. just think about how it would impact what you do after this point and if it has no negatives, all good. Also, since so close to harves, i wouldn't react ot the deficiency at this point. Just stick to water-only since i don't see any major fade. You can save a bit of effort the last 7 days or so. don't do the esoteric stuff at end of harvest -- like ice water or not watering or 36 hours of darkness etc. None of that stuff is proven to be effective and is just a blind stab in the dark. Leaving light off on harvest day isn't a bad idea, but it doesn't need to be dark for any extended period of time. You want to harvest in morning before lights cause plant to matabolize nutrients before you cut it down for drying, just as you want it dark where it is drying -- same reasons as it's alive for a while after you cut it. If you stagger your harvest, i don't know what's best in this regard. tidbit -- i cut 1 day early from previous trichome observances (clones). It really does develope more after you cut it down. Check with loupe after 2 days and compare trichomes of cut / drying buds.